A group of confederate prisoners escape to Canada and plan to rob the banks and set fire to the small town of Saint Albans in Vermont. To get the lie of the land, their leader spends a few days in the town and finds he is getting drawn into its life and especially into that of an attractive widow and her son.

On 26 September 1864, seven Confederate prisoners being held at a Union stockade in Plattsburg, New York, near the Canadian border, attempt to escape. As they open the gate, the hot-headed Lt. Keating shoots a Union soldier, and during the return fire, Capt. Lloyd Henderson is injured. Because Henderson is slowing them down, Maj. Neal Benton, the group's leader, decides to leave him behind, much to the dismay of Capt. Frank Dwyer. The group reaches Montreal, and soon after, Neal returns to the United States, to the small town of Saint Albans, Vermont. There, Neal, posing as a prosperous merchant named Neal Swayze, ingratiates himself with banker Josiah Anderson by claiming he wants to buy land and set up a business. Neal then obtains lodgings at a boardinghouse run by Katy Bishop, a war widow and the mother of a young son, Larry. Neal has difficulties controlling his temper when confronted with the townspeople's dispargements of the South and the Confederate Army, but nonetheless makes friends with Katy and Larry. Unaware that he is secretly charting the territory, Katy gives Neal a tour of the town and the surrounding farms, much to the chagrin of Union Army recruiter Capt. Lionel Foster, a wounded veteran who has loved Katy since childhood. Later, on 12 October, Neal returns to Montreal, where he meets with his compatriots and begins organizing their plan to raid Saint Albans. Hoping to relieve Southern troops by forcing Union soldiers to protect the northernmost states, Neal and Col. Tucker have gathered other escaped Confederate prisoners for the burning and looting of Saint Albans. Neal shows the men a mockup of the town, through which runs only one road, leading to the Canadian border. On the outskirts of town, however, lies a little-used road that also leads to the border, along which Neal has located an abandoned farmhouse. Neal plans to burn the town square and rob the banks, then retreat along the farm road, burning its wooden bridge behind them so that no one can follow. The Confederates hope that this will be the first of many raids, and so Neal assembles his men from the volunteers. Neal is reluctant to include Keating in the mission but is swayed by his promise to obey orders, and after Neal returns to Saint Albans, Robinson and Keating, posing as travelers, check into the hotel. Dwyer, pretending to be a peddler, also comes to town, while the other men move into the farmhouse, where they practice using the liquid fire bombs with which they will destroy Saint Albans. During one of their meetings, Neal prohibits the bloodthirsty Keating from shooting any civilians or burning any private homes, although Keating intimates that his attachment to the Bishops is the real reason for his order. On 17 October, moments before the raid is to begin, Neal is horrifed when two platoons of Union Cavalry unexpectedly ride into town. Knowing that they do not have sufficient numbers to defeat the Cavalry, Neal and Dwyer disperse their men and call off the raid. That night, a disappointed Keating disappears, while Neal is forced to attend a charity bazaar, at which captured Confederate "trophies" are auctioned. Neal attempts to outbid Foster for a Confederate flag, but the irate Katy wins it and later that night, gives it to Foster. Embarrassed, Foster admits to Katy that he is a coward who purposely got himself injured in order to avoid fighting, but the gentle Katy does not judge him. The following morning, Larry persuades Neal to accompany him to church, but the sermon is interrupted by a drunken Keating, who threatens to ignite the building with an explosive. Neal is forced to shoot Keating, whose Confederate identity is uncovered, and the townspeople acclaim Neal as a hero. Torn between his duty and his affection for the Bishops, Neal staggers out to the farmhouse, where he tells the men that the raid will take place the following day, after the Cavalry departs and before a regiment of Union infantry is scheduled to arrive. When Neal returns to the boardinghouse, he is met by a group of townspeople, who offer him a parcel of land in town as a reward for his actions in the church. Foster, who does not trust Neal, leaves in disgust, while Neal reluctantly accepts their gift. The following morning, after Neal has dressed in his Confederate uniform, he is found by Larry, who is heartbroken to discover that his hero is an enemy. Katy also finds Neal, who promises her that she will be safe, but before they can stop Larry, he runs away to find the departed Cavalry. Upon Neal's signal, his men attack the town and begin to set fires and rob the banks. Despite having only one arm, Foster takes a stand in a burning building and succeeds in wounding Neal and killing several others before being captured. Katy and the other citizens applaud Foster's bravery, and are heartened to learn from the Confederate lookout that Larry reached the Cavalry, which is quickly riding back to town. Neal forces the citizens to sit at the entrance of the town as a human blockade, but, fearing that Larry might get hurt, forbids his men to fire upon the Union soldiers. As Neal's men race off with their wagons loaded with money, Neal and Dwyer set other wagons on fire to hinder the arriving Cavalry. Neal and Dwyer then ride off with the soldiers in pursuit, but are able to set fire to the bridge in time to escape. Back in town, while Larry cries in his mother's arms, Katy remembers when Neal once asked her if she would understand if Confederate soldiers destroyed her town, as so many Southern towns had been destroyed by Union forces.